Concrete is a composite construction material composed primarily of aggregate, cement, and water. There are many formulations, which provide varied properties. The aggregate is generally coarse gravel or crushed rocks such as limestone, or granite, along with a fine aggregate such as sand. The cement, commonly Portland cement, and other cementitious materials such as fly ash and slag cement, serve as a binder for the aggregate. Various chemical admixtures are also added to achieve varied properties. For example, to reinforce a concrete, fibres can be added, such as metal fibres, polymer fibres, organic fibres, asbestos fibres and the like. Water is mixed with the dry concrete mixture, which enables it to be shaped (typically poured or casted) and then solidified and hardened (cured, set) into rock-hard strength concrete through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, finally creating a robust stone-like material. Concrete can be damaged by many processes, such as the freezing of water trapped in the concrete pores.
Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, motorways/roads, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats.
One particular application of concrete is its use for casting the cement tube in bore hole drilling. The cement tube is used to withstand the pressure from outside the bore hole, which can be the hydrostatic outside pressure from the water the surrounding earth/soil. Under ordinary conditions, the outside pressure contracts the tube. As hydrated cement is a material, optimized to withstand pressure, the outside pressure does not damage the cement tube. Ordinary loads from inside the tube are usually not larger than the external forces, hence under standard conditions cracking is not a risk. However, under non-standard conditions, like a blow-out, the cement tube is strongly pressurized from the inside, and hence, a tension stress is exerted on the cement tube. This may lead to brittle cracking, and hence to an unwanted and potentially hazardous emission of gas, oil and bore hole liquids. Due to geometry of the cement tube, a reinforcement with reinforcement steel bars or meshed grids that can prevent such cracking is not possible.
Fibre reinforcement has become popular for concrete construction for conventional use during the last decade. Metallic, glass and polymer fibres are used to replace the steel bar or grid reinforcements. Such fibres provide tensional strength, and by distributing the load, the necessary cracks are minimized. Thus, a fibre-reinforced structure is more elastic and in addition does not limit the choice of geometry.
After drilling, the oil/gas production tube or water/steam injection tube are inserted. To increase the weight and by that, increase the counter pressure towards the oil/gas well, a completion fluid is filled into the hole. Most of the cross section is covered with the completion fluid. The completion fluids used usually contain chlorides. Usually, stainless steel tubes are used for the gas/oil or water/steam hose.
In cold areas, the surface buildings and the top of the cement tube are exposed to low temperatures and frost during construction and operation. Due to shale gas exploitation, this situation is becoming more actual than ever, as a number of potential shale resources are located in cold areas in Russia and Canada.
Casting and curing concrete in cold weather, in particular at or below a—sustained—freezing temperature is challenging. The most common problem is that concrete freezes and/or goes through freeze/thaw cycles before acquiring adequate strength during curing.
Within the context of this application, “cold weather” is defined when the following conditions exist for at least three consecutive days:                the average daily temperature falls below 4° C., and        the air temperature does not rise above 10° C. for more than half a day in any 24-hour period.        
At said cold weather conditions, water starts to freeze in capillaries of concrete at −2° C., it expands up to 9% of its volume when it freezes causing cracks in the concrete matrix, and up to 50% of compressive strength reduction may occur if concrete freezes before reaching at least a compressive strength of 500 psi.
Casting concrete in cold weather follows the recommendations by ACI (American Concrete Institute) Guideline 306R-88. Insulation of the cast concrete, the use of setting accelerators (SA) and of water-reducing agents, also known as superplasticizer (SP), are described as measures to ensure a proper curing of the concrete.
A widely known approach is to add sodium nitrate to the concrete at dosages of up to 5 weight % relative to the concrete composition, comprising at least aggregate, cement, and water. This approach usually delivers quick-setting cement. U.S. Pat. No. 5,296,028 (Charles J. Korhonen et al., 1994) discloses an antifreeze composition consisting of sodium nitrate and sodium sulphate at a ratio of 3:1, wherein the antifreeze composition is present in the concrete at a dosage of 2 weight % to 8 weight %, relative to the weight of the concrete composition. However, the high alkali addition due to sodium increases the risk of alkali-aggregate-reactions (AAR) and in addition, sodium nitrate is known to significantly reduce compressive strength. Hence, this kind of concrete has a reduced durability, especially when it comes to freeze/thaw-resistance.
Some commercially available products combine several components in one admixture, such as a superplasticizer (SP) and a setting accelerator (SA). Water reduction using a superplasticizer (SP) is a common technique to reduce free water and increase salinity of the pore fluids (which also reduces the freezing point of water). For instance, U.S. Pat. No. 5,176,753 (John W. Brook, 1993) or the equivalent patent GB 2,195,328 (Sandoz, John W. Brook, 1993) describes the combined use of (1) a mineral freezing point depressant, for example calcium nitrate, (2) a superplasticizer, for example the sodium salt of naphthalene sulphonate-formaldehyde resin, (3) an inorganic set accelerator, for example sodium thiocyanate, and (4) an organic set accelerator, for example tetra (N-methylol) glycoluril.
In order to obtain a very quick setting of the concrete, the prior art literature indicates that trivalent ions like aluminium (Al3+) or iron (Fe3+) might be beneficial. This is documented especially for shotcrete (concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique). U.S. Pat. No. 4,444,593 discloses ferric nitrate blends for rapid setting. WO97/36839 (Tjugum, 1997) discloses aluminium-based salts, in particular aluminium nitrate. Shotcrete is not linked to cold weather concreting, as the concrete is, for example, applied in tunnels where no cold weather conditions prevail, in particular no temperatures below the freezing point of water.
Harald Justnes in Concrete, Volume 44, Number 1, February 2010 “Calcium nitrate as a multi-functional concrete admixture”, discloses the use of calcium nitrate as a set accelerator when used with a plasticiser counteracting the retardation by the plasticiser while maintaining rheology, as long-term strength enhancer, in anti-freeze admixtures or winter concreting admixtures, and as a corrosion inhibitor for the protection of embedded steel.
Standards are available describing how to cast concrete that needs to have increased freeze-thaw-resistance, for instance by adding an air-entraining-admixture (AEA).
However, there is still a need for an admixture that ensures a quick and sufficient hydration of a metallic fibre-reinforced cementitious composition and that improves the long term behaviour, mechanical strength and the resistance to corrosion and erosion of the cementitious solid comprising said admixture. In particular, such admixture can be used for casting the cement tube in drilling operations in cold weather conditions, in particular until the depth where the temperature reduces the metal tension.